I understand that spell damage in based on weapon damage, but how? A weapon has both a damage attribute (e.g., 2-3 damage), and a damage-per-second attribute (e.g., 3.0 damage per second).

Let's say we have two scenarios:

You equip an apprentice's wand: 3.0 Damage Per Second, 2–3 Damage, 1.20 Attacks per Second

You equip a variation of this wand: 3.0 Damage Per Second, 1-1.5 Damage, 2.40 Attacks per Second.

Now if you cast a Magic Missile (110% weapon damage) in scenario one, would it deal 2.2-3.3 damage? And 1.1-1.65 damage in scenario two?

So this could be balanced by the fact that weapons with higher attacks per second allow you to cast spells faster. But in this case, it would cost more arcane power. Is arcane power adjusted with attacks per second? Is this mechanism explained somewhere ?

I don't know how to edit the bounty, so I'll say this in the comments: Raven Dreamers answer seems good, but it has no sources whatsoever. I'd like this confirmed with sources or another answer with more complete reasoning.
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heisheMay 17 '12 at 16:30

Weapon speed doesn't come into play at all when calculating damage from abilities.

What this means is that weapon speed is balanced as a stat; either you deal more damage per hit (by picking a slower weapon), or you use more abilities (since weapon speed affects animation time). DPS is entirely useless as a comparison metric unless you're only auto-attacking.

"Weapon speed doesn't come into play at all when using abilities" - well, it affects how quickly you can perform the abilities, so I wouldn't say it doesn't come into play at all. It's just not used for damage calculation.
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OakApr 26 '12 at 22:08

I don't this this is explained anywhere, and I haven't heard of anyone doing enough testing to figure it out yet (I certainly haven't). This is something I'd like to know too.

I believe your inclinations are correct: it would cost more Arcane Power to use a higher attack per second wand.

Here's how I suspect it works:

Faster attack speed makes you attack faster with all abilities

Higher attack damage makes you hit harder with all abilities

Two weapons with the same DPS but different attack speeds will cost different amounts of power (Mana/Hatred/Spirit/etc.). There is no adjustment for speed.

In addition, most characters (Monk, Demon Hunter, Barbarian) have power-generating abilities. Attacking faster with those abilities means faster power generation, which would offset the fact that attacking faster with power-spending abilities makes you spend power faster.

I don't have a good source for this, but I am inferring based on my experience, my thoughts on how I would do it if I were designing the game, and the Monk ability Fists of Thunder which specifically states that it is inherently faster and therefore generates more Spirit than other stuff.

The power-generating abilities aside, straight damage values might make it seem like stronger, slower weapons be strictly better because they cost less to put out the same damage as fast weapons. However, damage isn't everything. When you attack slower, it takes you longer to cast each spell. You can't react as quickly, and you might miss more often due to enemies moving around. It's harder to take a swing (or shoot some lightning, or launch a kick) and move afterward. Try out a Barbarian with some giant axe and you'll feel a definite difference.

I have a feeling that mobility is going to be important, especially in harder difficulties. This would mean that being able to poke quickly and change position, or get a bunch of power in a hurry, will be valuable. How valuable it is, though, you'll have to decide for yourself based on your own judgment and playstyle.

It is true that using a weapon with the same DPS but faster attack speed will use more resource. For pure damage per resource, you're better off with a slower attack. But you have to keep in mind some other factors.

Overdamage. If you're facing a monster with 100 HP, it doesn't matter whether you hit him for 100 or 200, he's still dead. With a faster attack speed, you can kill 3 monsters in the time it takes you to hit 2 with the slower, so if they're all one hit kills, the faster weapon is giving you a better effective DPS.

A faster weapon attack allows you to be more mobile. You can attack and then move away to avoid being hit. So while you may be doing less DPS, you're gaining other advantages.

The slowest weapons with the highest DPS are generally 2-handed. So you'll have to decide whether you want to maximize your damage per resource, at the cost of losing the ability to equip a second item (such as a shield).

I'm not sure how a faster attack allows you to be more mobile... does a slow attack speed actually slow down your attack animation as well? If that's the case then fast attack speed all the way.
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Sadly NotApr 20 '12 at 4:43

Magic Missile (110% weapon damage) will do 2.2-3.3 damage. You can cast it 1.2 times per second

Scenario 2:

You equip a variation of this wand: 3.0 Damage Per Second, 1-1.5 Damage, 2.40 Attacks per Second.

Magic Missile (110% weapon damage) will do 1.1-1.65 damage. You can cast it 2.4 times times per second

Notice that for spells that cost arcane power, the spell cost is not adjusted, so faster attacking weapons "consume" more arcane power for the same DPS.

Advantages of fast attacking weapons (wands 1.4, daggers 1.5):

Better for "per hit" or "per critical hit" abilities, such as AP per crit

Advantages of slow attacking weapons (2h maces 0.9, staves 1.0):

Easier to to find. (even when factoring in the off-hand)

Consumes less resource

Better for kiting

Better for damaging cool-down abilities (for example, wave of force)

Another fact that I found very surprising: The weapon speed changes only the speed of offensive abilities. For example, I can cast 4 teleport with wormhole regardless of my weapon speed. So I thought that fast attacking weapons would give you an advantage for casting defensive spells, but this is not true. The casting speed of defensive and ability spells is always the same.

So it's not a surprise that my build involves a 2handed mace and no signature spell. I am currently running through hell solo and it's very smooth. I expect to end hell with this build without any difficulty whatsoever. (expect for a few very bad-rolled rare packs). Edit: Now I am soloing inferno (i have adjusted my build). The first act is very easy. The only problem are bad rolled rare packs. I have to avoid about a third of them.

The basic attacks for all classes, like magic missile, take your base attack speed into account for their usage; they essentially scale directly off DPS.

All OTHER skills take only weapon base damage + modifiers into account, NOT DPS.

With most ability based builds (especially the wizard) you're best off going with a strong, high damage 2hander.

With builds that focus a lot on the basic attacks, or using a lot of defensive/utility skills, you're better off dual weilding for the extra stats and attack speed, or shieldweilding for the added armor. Shield block adds some notable reduction against swarmers, but those aren't usually the problem mobs.

EDIT: There may be a few exceptions to this, but it's the general rule. This is from my experience with two normal and half a nightmare playthrough.

Conclusion
If you use as main dmg source something like the fuuucking bears of the WD
and you have +Fix dmg on Mojo, rings and amulet you can have better DPS but still a high mana consumation that can be fill with some passiv like + 300% regen :-D

Basically the AP cost is not scaled by weapon speed, so a 2H weapon makes more damage per AP.
However the Blizzard guy says that generally a 2H weapon only makes 15%-20% more damage than 1H and this gap can be filled using orbs. So in the end a 1H weapon can make the same damage AND give you extra speed and better spilled damage.

My English is a bit rusty and I don't get the sentence: "In Diablo, virtually every spell is working off of your weapon speed,". Does it mean that the faster the weapon speed, the faster your spell casting? Based on the response it seems like that, but if true it contradicts other people here, that tested it and discovered that you don't get extra speed for defensive spells (unless Teleport is an exception).

Wanna add that i experience the same thing atm with my demonhunter. Running with powerful % weapon dmg skills like Fan of Daggers (10sec cd) and Spike Trap Long Fuse really pays of with a heavy 2H Crossbow, 1 attack per second(of course you can pimp this with a quiver and more sources for the attack speed increase stat). Heavy Aoe damage is the result. The only downfall is indeed the fact that my hatred recharge is slower because i shoot the devouring arrow also at a slower rate (but for more dmg).